Nowadays, various spectacle lenses for vision correction are known. For example, several examples can be put forward, such as a single vision lens in which one region for seeing an area at a predetermined distance is formed, a single vision lens in which power changes as view departs from the region, and a progressive power lens (a progressive multifocal lens in this description) which includes a portion (so-called progressive portion), where power changes continuously with respect to these two lenses.
For example, a progressive power lens according to PTL 1 includes: a distance portion which has distance power for distance vision; a near portion which has near power for near vision; and a progressive portion which exists between the distance portion and the near portion. In the spectacle lenses according to PTL 1, the distance portion and the near portion each have a respective prism of which power is different from each other. A prism is provided as a prescription to correct visual symptoms of a user, such as strabismus, heterophoria and fixation disparity. Hereafter this prism is called a “prescription prism”. In paragraph [0004] of PTL 1, it is mentioned that double vision and poor depth perception may be experienced if the prisms of both eyes are incorrectly prescribed. An object of PTL 1 is to provide correctly prescribed prisms to progressive multifocal lenses, so that binocular vision can be comfortably exhibited in both the distance vision and near vision (paragraph [0005] in PTL 1).
Besides the above content, an element allowing the user to see an object clearly is the magnification by the spectacle lens (hereafter merely called “magnification”). Magnification by the spectacle lens refers to the state of an object directly in front of the user’ eyes that appears larger or smaller depending on the power of the spectacle lens. It is natural for the user wearing the spectacle lens to see an object at actual size. Therefore conventionally it is thought that the magnification by the spectacle lens is close to 1.
In the case of the progressive power lens according to PTL 1, a progressive portion, in which the power continuously changes, exists between the distance portion and the near portion. Therefore in a singular spectacle lens, the above mentioned problem of magnification is generated. This is mentioned in PTL 2 to PTL 4, for example.
PTL 2 mentions that the difference of the magnification in the distance portion and the magnification in the near portion causes fluctuation and distortion when the spectacle lens is worn. For this, PTL 2 discloses a technology to constitute the progressive surface on the eyeball side surface (so-called inner surface).
The technology according to PTL 3 is a technology for a double-sided progressive lens, of which the surfaces on the object side (so-called outer surface) and on the inner surface are both progressive surfaces, and according to this technology, curvature of the near portion on the outer surface in the horizontal direction is increased, so that the magnification difference between the vertical direction and the horizontal direction of the near portion is decreased, thereby the generation of distortion is controlled.
The technology according to PTL 4 is also a technology for a double-sided progressive lens, and according to this technology, the curvature of the near portion on the outer surface in the horizontal direction is decreased, so that the magnification difference between the distance portion and the near portion is decreased, while decreasing the magnification difference between the vertical direction and the horizontal direction of the near portion, thereby the generation of distortion is controlled.